This study uses daily diary data to investigate if and how parents and children affect the functioning of each other's hypothalamic-pituitary-adrenocortical (HPA) system. Close, positive relationships have strong implica- tions for child and parent health. One avenue by which relationships may influence health is their impact on stress-related physiology; specifically, the functioning of the HPA system. Cortisol, the hormonal byproduct of the HPA system, increases in response to stress, mobilizing energy resources. Chronic, repeated activation of the HPA system can trigger unhealthy cortisol patterns, which are linked to a host of long-term physical and mental health problems. Determining if and how parents and children affect one another's HPA functioning is crucial to promoting health over the life course. Recent research suggests children and parents not only influ- ence the other's HPA system but that parent and child's cortisol will also ?sync up? or covary over time. Yet, little is known about how parent?child cortisol covariation occurs, the direction of effects within the parent?child dyad, or the linkages between parent-child cortisol covariation and child outcomes. This study will fill gaps in these understudied areas by accomplishing the following 3 aims: Aim 1 tests the direction of the relations between parent and child daily cortisol and the relative strength of those associations. To address this aim, we will examine the bidirectional relationships between child and parent cortisol, testing the hypothesis that parental cortisol is linked to child cortisol patterns and vice versa. Aim 2 tests whether parent-child be- havioral interactions mediate the linkages between parent and child cortisol, unpacking the process un- derlying parent?child cortisol covariation. Mediators include positive/negative parent?child interactions and par- ent?child time together. Aim 3 tests if parent?child cortisol covariation is associated with child out- comes, such as risky behavior, depression, and physical health symptoms. Aim 4 tests if parent?child corti- sol covariation differs based on child age and biological sex. We will accomplish these aims using sec- ondary data from an 8-day daily diary study that was part of the Work, Family, Health Project, an NICHD study. Innovation: The daily diary design enables us to capture critical day-to-day fluctuations in cortisol and parent? child interactions. Measures of cortisol were collected from parents and children at multiple time points in a day, enabling us to establish the temporal ordering needed to investigate day-to-day bidirectional associations and mediational processes. The diversity of the sample will enable us to investigate key moderating mecha- nisms such as child sex and age. Our statistical approach will enable us to test whether parent?child cortisol covariation has health implications for children beyond behavioral interactions. Public Health Impact: Under- standing if and how parents and children affect one another's HPA functioning has important implications for long-term health outcomes. This study will advance our understanding of which family members, behaviors, and subgroups to target in interventions aimed to improve child and parent health.